do_wait: fix security checks

Imho, the current usage of security_task_wait() is not logical.

Suppose we have the single child p, and security_task_wait(p) return
-EANY.  In that case waitpid(-1) returns this error.  Why? Isn't it
better to return ECHLD? We don't really have reapable children.

Now suppose that child was stolen by gdb.  In that case we find this
child on ->ptrace_children and set flag = 1, but we don't check that the
child was denied.  So, do_wait(..., WNOHANG) returns 0, this doesn't
match the behaviour above.  Without WNOHANG do_wait() blocks only to
return the error later, when the child will be untraced.  Inho, really
strange.

I think eligible_child() should return the error only if the child's pid
was requested explicitly, otherwise we should silently ignore the tasks
which were nacked by security_task_wait().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Eric Paris <eparis@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -1142,10 +1142,14 @@ static int eligible_child(pid_t pid, int options, struct task_struct *p)
 		return 0;
 
 	err = security_task_wait(p);
-	if (err)
-		return err;
+	if (likely(!err))
+		return 1;
 
-	return 1;
+	if (pid <= 0)
+		return 0;
+	/* This child was explicitly requested, abort */
+	read_unlock(&tasklist_lock);
+	return err;
 }
 
 static int wait_noreap_copyout(struct task_struct *p, pid_t pid, uid_t uid,
@@ -1476,16 +1480,14 @@ static long do_wait(pid_t pid, int options, struct siginfo __user *infop,
 	DECLARE_WAITQUEUE(wait, current);
 	struct task_struct *tsk;
 	int flag, retval;
-	int allowed, denied;
 
 	add_wait_queue(&current->signal->wait_chldexit,&wait);
 repeat:
 	/*
 	 * We will set this flag if we see any child that might later
 	 * match our criteria, even if we are not able to reap it yet.
 	 */
-	flag = 0;
-	allowed = denied = 0;
+	flag = retval = 0;
 	current->state = TASK_INTERRUPTIBLE;
 	read_lock(&tasklist_lock);
 	tsk = current;
@@ -1498,13 +1500,8 @@ static long do_wait(pid_t pid, int options, struct siginfo __user *infop,
 				continue;
 
 			if (unlikely(ret < 0)) {
-				denied = ret;
-				continue;
-			}
-			allowed = 1;
-
-			retval = 0;
-			if (task_is_stopped_or_traced(p)) {
+				retval = ret;
+			} else if (task_is_stopped_or_traced(p)) {
 				/*
 				 * It's stopped now, so it might later
 				 * continue, exit, or stop again.
@@ -1544,22 +1541,24 @@ static long do_wait(pid_t pid, int options, struct siginfo __user *infop,
 		}
 		if (!flag) {
 			list_for_each_entry(p, &tsk->ptrace_children,
-					    ptrace_list) {
-				if (!eligible_child(pid, options, p))
+								ptrace_list) {
+				flag = eligible_child(pid, options, p);
+				if (!flag)
 					continue;
-				flag = 1;
-				break;
+				if (likely(flag > 0))
+					break;
+				retval = flag;
+				goto end;
 			}
 		}
 		if (options & __WNOTHREAD)
 			break;
 		tsk = next_thread(tsk);
 		BUG_ON(tsk->signal != current->signal);
 	} while (tsk != current);
-
 	read_unlock(&tasklist_lock);
+
 	if (flag) {
-		retval = 0;
 		if (options & WNOHANG)
 			goto end;
 		retval = -ERESTARTSYS;
@@ -1569,8 +1568,6 @@ static long do_wait(pid_t pid, int options, struct siginfo __user *infop,
 		goto repeat;
 	}
 	retval = -ECHILD;
-	if (unlikely(denied) && !allowed)
-		retval = denied;
 end:
 	current->state = TASK_RUNNING;
 	remove_wait_queue(&current->signal->wait_chldexit,&wait);